Meet Reiji, the secret party member in the original Persona.
Meet the checklist of shit you gotta do to unlock him.
the following:
After you have cleared the Transformed Mikage Hospital (異界化御影総合病院):
- Talk to the teacher in the Teacher's Lounge (職員室) next to the 1F Entrance Hall, and learn about the student hanging out in the Empty Classroom on the 2F.
- Go to the Empty Classroom (空教室) to the west furthest to the back at 2F, meet Reiji for the first time.
- Go talk to the male student (男子生徒) in Class 2-1.
- Head to the convenience store at Joy Street (ジョイ通り), talk to Reiji's mother (レイジ母). Tell her that (i) you know about her son (知っている) and (ii) you will be happy to be his friend (いいよ). Only talk to her once.
- Head to the casino in the same mall, and talk to the blonde man (チーマー). He will tell that Reiji likes to wander at the Abandoned Factory.
- Go to the Abandoned Factory (廃工場) and meet with Reiji. He will just leave instantly.
- Head down to the SEBEC Building (セベクビル), there will be some guards blocking the road. See the event involving Reiji.
- Keep playing with only four members (MC, Maki, Mark, and Nanjo). Whenever other members (Brown, Elly, and Ayase) propose to join, decline.
- Eventually the MC's school will become one of the dungeons, meet with Reiji on the 2F and he will join forcefully.
You have to basically go out of your way to explore early in the game(Persona 1's exploration not being that great IMO doesn't help) talk to an otherwise non descript NPC(only once mind you, try talking to her a second time and you apparently lock yourself out off the character) and then tell every other potential party memember to leave also meaning you go without a fifth team member much longer.
I'm sure there's secret characters much worse somewhere bu Reiji always stick out to me. He gets a bit more story so it's still worth it since P1 isn't THAT hard but still how was anyone supposed to do this without a guide I have no idea.